1. Field of the Invention
This invention relates to color-developing agents for recording materials (for example, pressure-sensitive copying paper). In particular, the present invention provides color-developing agents excellent in initial and ultimate color-developing capacities and also in waterproofness. Further, this invention also pertains to color-developing sheets making use of these color-developing agents.
2. Description of the Related Art
Salicylic acid derivatives, for example, metal salts of 3,5-di-substituted salicylic acid derivatives are conventionally known to be useful as color-developing agents for pressure-sensitive copying paper. Each 3,5-di-substituted salicylic acid derivative is produced from its corresponding 2,4-di-substituted phenol derivative and carbon dioxide, namely, by using the Kolbe-Schmitt reaction (see, for example, U.S. Pat. No. 3,983,292). However, use of a metal salt of a 3,5-di-substituted salicylic acid derivative obtained in this manner, for example, zinc 3,5-di(.alpha.-methylbenzyl)salicylate, as a color-developing agent for pressure-sensitive copying paper creates a problem such that developed color images fade away when exposed to water.
As measures for overcoming this problem in physical property, the following production processes have already been proposed. Namely, a salicylate ester and styrene are reacted. Subsequent to hydrolysis of the resulting salicylate ester resin, a multivalent metal compound is caused to act to produce the multivalent metal salt of the salicylic acid resin. For example, styrene is reacted with methyl salicylate in the presence of concentrated sulfuric acid to obtain a methyl salicylate resin, followed by hydrolysis in an aqueous alkaline solution. A multivalent metal compound (e.g., zinc sulfate) is then caused to act, whereby the multivalent metal salt of the salicylic acid resin is produced (U.S. Pat. No. 4,952,648). As an alternative, a salicylic acid and a styrene are reacted at 50.degree. to 150.degree. C. in the presence of an acid catalyst, followed by the reaction with a metal salt of a fatty acid so that the metal salt of a high molecular salicylic acid resin is produced (U.S. Pat. No. 4,929,710). Color-developing agents obtained by these processes, however, are still unable to bring about an improvement in both initial and ultimate color-developing capacities.
A color-developing agent has been proposed recently, which features inclusion of (A) a multivalent metal salt of a salicylic acid derivative and (B) a phenol substituted by styrene or the like or a metal salt thereof as essential components (Japanese Patent Laid-Open No. 286304/1994). However this color-developing agent is also still insufficient in color-developing capacity.